This invention relates to a method for the substantially continuous deposition of polycrystalline silicon ribbon. More particularly, this invention relates to a highly efficient, cost effective method for the substantially continuous deposition of polycrystalline silicon ribbon on foils from which the ribbon can be removed by shear separation.
The semiconductor industry uses large quantities of high quality monocrystalline silicon in the form of thin sheets or wafers. The need for this type of silicon is growing rapidly, especially with the increasing interest in the use of solar cells for the production of electrical power. Most silicon wafers are produced by growing a monocrystalline silicon ingot, sawing the ingot into thin wafers, and then lapping and polishing the wafers to the desired thickness and surface finish. This is an extremely costly process. While the cost may be acceptable for the production of transistors, integrated circuits, and the like, the cost of these wafers is unacceptably high if solar cells are to be competitive for the large scale production of electricity.
Other techniques have been developed for the production of thin sheets of silicon suitable for the production of solar cells. One of these is the so-called ribbon-to-ribbon (RTR) growth process which is expected to yield high quality substrates for use in solar cell fabrication as well as for use in the production of a variety of other semiconductor products. The RTR process uses a scanned laser beam to locally melt a polycrystalline ribbon and to induce crystal growth as the ribbon is translated through the laser beam. Thus a polycrystalline ribbon is transformed into a macrocrystalline ribbon. A macrocrystalline structure is one in which the crystals are of sufficiently large size to permit efficient semiconductor action. Therefore a monocrystalline ribbon wherein the ribbon is but a single crystalline body is encompassed within the term macrocrystalline. In this context the word "ribbon" implies a long strip having a width much greater than its thickness. Typical dimensions might be a length of several meters, a width of 1 to 10 centimeters and a thickness of 50 to 250 micrometers. Although the RTR process has proved to be viable for the production of macrocrystalline substrates, a fundamental stumbling block to the overall process has been that of supplying the required polycrystalline silicon ribbon starting material. A number of techniques have been proposed for solving this problem, but none have proved highly successful. One technique, for example, takes silicon powder, compacts it to the desired shape, and then fuses it into a solid body by scanning with an electron beam or a laser beam. Another technique involves the chemical vapor deposition of polycrystalline silicon on a foreign substrate. As practiced theretofore, however, this process has been unacceptable since it is a discontinuous process carried out in a conventional RF heated furnace. The process is inefficient and expensive because of the method of heating used, the incomplete reaction of the gases employed, the deposition of silicon on other than desired surfaces, and the discrete as opposed to continuous nature of the process.
Accordingly, it is an object of this invention to provide a method for the substantially continuous, cost effective growth of polycrystalline silicon ribbon.
It is a further object of this invention to provide a method for the growth of epitaxial layers on monocrystalline silicon substrates.